Recently, as portable electronic products, including camcorders, mobile phones, and notebook computers, are required to be smaller and lighter and to have increased functionality, the demand for high-performance small batteries, which are capable of being used for an extended time and are lightweight and highly reliable, is increasing. Corresponding to such demand, lithium secondary batteries are receiving great attention.
The lithium secondary battery is classified into, depending on the type of electrolyte, a lithium metal battery, a lithium ion battery, and a lithium polymer battery, and further into, depending on the type of package for sealing an electrode assembly, a rectangular battery, a cylindrical battery, and a pouch battery. The pouch battery is mainly used for lithium polymer secondary batteries, and the fabrication method thereof follows.
That is, positive and negative electrode active materials are applied on both surfaces of a collector to manufacture electrode plates, between which a separator is then interposed and stacked, after which a bicell lamination process is carried out, thus manufacturing an electrode assembly. Subsequently, a plasticizer is extracted from the electrode assembly, taps are welded to leads of the electrode assembly, and then the electrode assembly is housed in a pouch case. After the electrode assembly is housed in the pouch case, an electrolytic solution is introduced into the electrode space in the battery case such that the electrode assembly is impregnated with the electrolytic solution.
After the completion of the introduction of the electrolytic solution, the open edge of the pouch is covered and is then primarily sealed. Subsequently, an aging process for stabilizing the battery is conducted, after which pre-charging is conducted to a state-of-charge (SOC) of 10% or less. The pre-charging process prevents the battery case from breaking due to the gassing in the battery case in the event of over-charge of the battery or the like. Through the pre-charging process, the gas is formed in the pouch case. The gas thus formed is removed through an outlet which is open or cut, and the gas outlet is thermally fused and is thus sealed. The process of exhausting the gas from the electrode space and thermally fusing the gas outlet is typically referred to as degassing. These days, as a styrene butadiene rubber/carboxyl methyl cellulose (SBR/CMC) system, which is a water-based binder for a negative electrode, is applied to rectangular batteries, the degassing process may be chiefly conducted, or a process of adding a filling liquid in several separate stages may be applied.
In the lithium secondary battery, at the time of pre-charging, lithium ions from lithium metal oxide used for a positive electrode are moved to a carbon (crystalline or amorphous) electrode, serving as a negative electrode, to thus be inserted into the carbon of the negative electrode. Then, the lithium ions react with the carbon negative electrode, thus forming Li2CO3, Li2O, and LiOH, which are then formed into a thin film, called an SEI (solid electrolyte interface), on the surface of the negative electrode. Such an SEI has an influence on the movement of ions and electric charges to thus result in changes in the performance of the battery. The properties of the film are known to be greatly changed depending on the type of solvent used in the electrolytic solution and the properties of the additive.
When the lithium secondary battery is continuously used for an extended time, or is left to stand at high temperatures, a swelling phenomenon, in which the battery swells due to gassing, occurs. The amount of as that is generated is known to be dependent on the state of SEI. Hence, in order to prevent the swelling phenomenon at high temperatures, techniques for inducing the stable formation of the SEI are required. Further, known are methods of improving the high-temperature lifespan of the lithium secondary battery by adding a negative electrode SEI improver, such as LiF2BC2O4, 3,9-divinyl-2,4,8,10-tetraoxaspiro[5,5]undecane, LiB(C2O4)2, poly(ethyleneglycol)borate and derivatives thereof, halogen-substituted carbonate, and vinyl silane, to the electrolyte.
However, because the aforementioned negative electrode SEI improver has lower reducibility than general additives such as VC (vinylene carbonate) or VEC (vinyl ethylene carbonate), reduction may be conducted only when the voltage of the negative electrode is further decreased. Thus, when pre-charging to SOC of 10% or less, and then degassing as in the conventional degassing process are conducted, the additive is not 100% reduced, and thus gas is additionally generated in a subsequent formation process, undesirably causing swelling problems.